1. Field of the Invention
The present invention relates to an interferometer and more particularly to an optical interferometer for measuring the surface accuracy of the spherical surface of an object to be measured with a very high degree of accuracy.
2. Description of the Prior Art
In the past, interferometers of the type designed to produce interference fringes by utilizing the interference of light and to analyze the interference fringes to measure the physical quantity of the surface of an object to be measured have been used for measuring the accuracy of the spherical surface of an optical element such as a lens or mirror and typical types of these interferometers include for example the Twyman-Green interferometer and the Fizeau interferometer.
For instance, the Twyman-Green interferometer is designed so that after the source light of an extremely narrow spectral range emitted from a laser has been expanded through a beam expander into parallel rays of a suitable beam cross-sectional area, the light beam is divided into two parts or directions by a beam splitter thus utilizing one of the two light beams for measuring purposes and the other light beam for reference purposes. The measuring light beam is diverged by a lens and projected as a spherical wave onto the surface to be measured (e.g., the concave spherical surface) from which the reflected light beam travels back through the same optical path as previously and converted back into parallel rays and returned to the beam splitter through the lens. The reference light beam is reflected from a reference reflecting surface (an ideal spherical surface to serve as a reference) and returned as a light beam having a wave front of a required shape to the beam splitter. The measuring light beam reflected from the spherical surface to be measured and the reference light beam converted to have the wave front of the required shape are superposed one upon another by the beam splitter and the resulting composite light beam forms an image of the light source on a two-dimensional detector through another lens. At this time, interference fringes are produced on the detector due to the difference in optical path between the two light beams. The light and dark condition of the interference frings is read by the detector and is processed by a computer, thus computing a shape error of the surface to be measured. In other words, if the surface to be measured includes any portion distorted with respect to the reference reflecting surface, the distortion is analyzed to measure a shape error of the surface to be measured which is based on the reference reflecting surface.
On the other hand, in order to improve the measuring accuracy, the reference reflelcting surface is vibrated in the direction of the optical axis by a piezoelectric element or the like and thus the apparatus is operated as an AC interferometer thereby improving the accuracy of measurement.
Also, in order to eliminate the effect of the aberration of the optical system in the optical path during the measurement of a concave or convex surface, an attempt has also been made in which a spherical surface gage (a spherical surface whose surface accuracy is known) is preliminarily set in place of a surface to be measured thereby measuring the surface accuracy and the measured value of the actually measured surface is calibrated in accordance with the measured surface accuracy. In this case, if .lambda. represents the wavelength of the measuring light, the absolute accuracy of the spherical surface gage itself which is used for calibricating the effect of the aberration of the optical system is on the order of .lambda./40 (.lambda.=633 nm).
However, recently the surface accuracy of not greater than .lambda./100 to .lambda./1000 (several tens .ANG.) has been required for a short wavelength optical element, particularly a soft X-ray optical element and therefore the conventional interferometers of the type using a spherical gage have had the disadvantage of being unable to meet the required accuracy.